1. Field of the Invention
The present invention relates to sewing machines and more particularly to a sewing unit for successive or repetitive production of a number of stitch groups in a single workpiece. For example, the unit is applicable to the sewing of successive groups of buttons or buttonholes, where an automatically operated device is adapted to sew such buttons or buttonholes which are arranged in a group on a workpiece, which device in turn is mounted in a frame in a sewing machine.
A carriage which is supported on a frame in a sewing machine is intermittently displaceable in opposite directions. The carriage carries a stitch group sewing machine which is equipped with a liftable material presser. The carriage is provided with a clamping device which is located in the zone of operation of the sewing elements for the stitch group to temporarily hold the workpiece positively clamped in a position. A feed device is provided having a mark for aligning, receiving and placement of the workpiece that is to be sewn. A removal device is provided for removing a sewn workpiece from the clamping device. A control, which is preferably electropneumatic, has a drive unit which periodically drives the stitch group sewing machine in a selected direction, back or forth, by means of a tension transmission. The clamping device is slidably supported on the frame and can be periodically engaged by the carriage by means of a clutch unit. A pushing device is also provided that pushes the clamping device in a direction opposite to its previously executed movement into an initial operating position after the completion of the stitch group sewing operation.
2. Background Art
Sewing units for successive production of a number of stitch groups in a workpiece are known, for instance in German Patent 26 50 334. In such sewing units, the buttonholes near the front edge of a shirt front or blouse front are held under tension and are sewn in an efficient manner because they are over-lapped. For this purpose, the stitch group sewing machine is moved between two sewing operations along the essentially stationary clamping device from a position at the buttonhole that has just been sewn up to a position at which the next buttonhole is to be sewn. The stitch group sewing machine is thus moved intermittently in one direction up to the last buttonhole to be sewn. During the sewing of buttonholes, a workpiece that is subsequently to be sewn is manually placed in the feed device. As soon as the removal device has removed the previously sewn workpiece from the clamping device, the feed device automatically transfers the next workpiece to the clamping device. In order to be able to sew buttonholes on that next workpiece, the stitch group sewing machine is now moved in the opposite direction, relative to the above mentioned sewing operation. While it is sewing the first workpiece, if the stitch group sewing machine is moved, for instance, in the direction U, then while it is sewing the next sewing piece, it moves in the direction V. The reversal of motion of the stitch group sewing machine avoids idle strokes. Furthermore, overlapping by placing the second workpiece in the feed device while the first workpiece is being sewn, high productivity is achieved in a known sewing unit.
However, the known unit has the disadvantage that the above described movement of the stitch group sewing machine from buttonhole to buttonhole is executable only between two sewing operations, so that the time required for said motion must be described as unproductive idle time.